Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene, and carbon nano-tubes. The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and as composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production rates commonly associated with all of the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Rather than trying to discover much lower-cost processes for nano-tubes, we have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but can be produced in larger quantities and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called nano-scaled graphene plates (NGPs). NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. Studies on the structure-property relationship in isolated NGPs could provide insight into the properties of a fullerene structure or nano-tube. Furthermore, these nano materials could potentially become cost-effective substitutes for carbon nano-tubes or other types of nano-rods for various scientific and engineering applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGP will be available at much lower costs and in larger quantities.
Direct synthesis of the NGP material had not been possible, although the material had been conceptually conceived and theoretically predicted to be capable of exhibiting many novel and useful properties. In a commonly assigned patent, one of the present inventors (Jang) and our colleague (Huang) have provided an indirect synthesis approach for preparing NGPs and related materials [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. In most of the prior art methods for making separated graphene platelets, the process begins with intercalating lamellar graphite flake particles with an expandable intercalation agent (also known as an intercalant or intercalate) to form a graphite intercalation compound (GIC), typically using a chemical oxidation [e.g., References 1-5, listed below] or an electrochemical (or electrolytic) method [e.g., Refs.6, 7, 17, 18]. The GIC is characterized as having intercalate species, such as sulfuric acid and nitric acid, residing in interlayer spaces, also referred to as interstitial galleries or interstices. In traditional GICs, the intercalant species may form a complete or partial layer in an interlayer space or gallery. If there always exists one graphene layer between two intercalant layers, the resulting graphite is referred to as a Stage-1 GIC. If n graphene layers exist between two intercalant layers, we have a Stage-n GIC.) This intercalation step is followed by rapidly exposing the GIC to a high temperature, typically between 800 and 1,100° C., to exfoliate the graphite flakes, forming vermicular graphite structures known as graphite worms. Exfoliation is believed to be caused by the interlayer volatile gases, created by the thermal decomposition or phase transition of the intercalate, which induces high gas pressures inside the interstices that push apart neighboring graphene layers or basal planes. In some methods, the exfoliated graphite (worms) is then subjected to air milling, air jet milling, ball milling, or ultrasonication for further flake separation and size reduction. Conventional intercalation and exfoliation methods and recent attempts to produce exfoliated products or separated platelets are discussed in the following representative references:                1. J. W. Kraus, et al., “Preparation of Vermiculite Paper,” U.S. Pat. No. 3,434,917 (Mar. 25, 1969).        2. L. C. Olsen, et al., “Process for Expanding Pyrolytic Graphite,” U.S. Pat. No. 3,885,007 (May 20, 1975).        3. A. Hirschvogel, et al., “Method for the Production of Graphite-Hydrogensulfate,” U.S. Pat. No. 4,091,083 (May 23, 1978).        4. T. Kondo, et al., “Process for Producing Flexible Graphite Product,” U.S. Pat. No. 4,244,934 (Jan. 13, 1981).        5. R. A. Greinke, et al., “Intercalation of Graphite,” U.S. Pat. No. 4,895,713 (Jan. 23, 1990).        6. F. Kang, “Method of Manufacturing Flexible Graphite,” U.S. Pat. No. 5,503,717 (Apr. 2, 1996).        7. F. Kang, “Formic Acid-Graphite Intercalation Compound,” U.S. Pat. No. 5,698,088 (Dec. 16, 1997).        8. P. L. Zaleski, et al. “Method for Expanding Lamellar Forms of Graphite and Resultant Product,” U.S. Pat. No. 6,287,694 (Sep. 11, 2001).        9. J. J. Mack, et al., “Chemical Manufacture of Nanostructured Materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005).        10. M. Hirata and S. Horiuchi, “Thin-Film-Like Particles Having Skeleton Constructed by Carbons and Isolated Films,” U.S. Pat. No. 6,596,396 (Jul. 22, 2003).        11. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” Pending, U.S. patent Ser. No. 11/509,424 (Aug. 25, 2006).        12. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Mass Production of Nano-scaled Platelets and Products,” Pending, U.S. patent Ser. No. 11/526,489 (Sep. 26, 2006).        13. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Method of Producing Nano-scaled Graphene and Inorganic Platelets and Their Nanocomposites,” Pending, U.S. patent Ser. No. 11/709,274 (Feb. 22, 2007).        14. Aruna Zhamu, JinJun Shi, Jiusheng Guo, and Bor Z. Jang, “Low-Temperature Method of Producing Nano-scaled Graphene Platelets and Their Nanocomposites,” Pending, U.S. patent Ser. No. 11/787,442 (Apr. 17, 2007).        15. Aruna Zhamu, Jinjun Shi, Jiusheng Guo and Bor Z. Jang, “Method of Producing Exfoliated Graphite, Flexible Graphite, and Nano-Scaled Graphene Plates,” Pending, U.S. patent Ser. No. 11/800,728 (May 8, 2007).        16. Aruna Zhamu, Joan Jang, Jinjun Shi, and Bor Z. Jang, “Method of Producing Ultra-thin, Nano-Scaled Graphene Plates,” Pending, U.S. patent Ser. No. 11/879,680 (Jul. 19, 2007).        17. N. Watanabe, et al., “Method of Producing a Graphite Intercalation Compound,” U.S. Pat. No. 4,350,576 (Sep. 21, 1982).        18. R. A. Greinke, “Expandable Graphite and Method,” U.S. Pat. No. 6,406,612 (Jun. 18, 2002).        19. L. T. Drzal, H. Fukushima, B. Rook, and M. Rich, “Continuous Process for Producing Exfoliated Nano-Graphite Platelets,” U.S. patent application Ser. No. 11/435,350 (May 16, 2006).        20. L. T. Drzal and H. Fukushima, “Expanded Graphite and Products Produced Therefrom,” U.S. patent application Ser. No. 11/363,336 (Feb. 27, 2006); Ser. No. 11/361,255 (Feb. 24, 2006); Ser. No. 10/659,577 (Sep. 10, 2003).        
In a nano-scaled graphene platelet (NGP), the largest dimension is defined as the length, the smallest dimension as the thickness, and the third or intermediate dimension as the width. It is possible to have length and width being identical or comparable in size. Prior art methods have successfully provided NGPs with a high length-to-thickness ratio, but with comparable length and width dimensions (length-to-width ratio being close to unity). No prior art method has been focused on the consistent production of NGPs with a high length-to-width ratio (greater than 3, preferably greater than 5, and most preferably greater than 10). In several industrial applications, to be discussed later, it is highly desirable to have NGPs with a high length-to-width ratio, in addition to a high length-to-thickness ratio.